The Spamhaus DROP list – “DROP (Don’t Route Or Peer) and EDROP are advisory “drop all traffic” lists, consisting of stolen ‘hijacked’ netblocks and netblocks controlled entirely by criminals and professional spammers.”

Basically, the DROP list is netblocks that you don’t want making connections to and from your network. All kinds of garbage emanates from these networks… spamming, scanning, harvesting, DNS-hijacking and DDoS attacks to name a few.